How to move multiple columns into rows

Dears,

I will try to give you as-is and the target I want to achieve:

AS-IS
M1 M2 M3
Item1 10 20 30
Item2 30 40 60
Item3 40 50 30

TO-BE

Item1 M1 10
Item1 M2 20
Item1 M3 30
Item2 M1 10
Item2 M2 20
....

Please help!! I have to reconcile such stuff each month and I'm getting crazy.